By "complexity" what do you mean? Complexity of sounds or complexity of meaning? I'm assuming you mean the former as the latter would imply that humpbacks are more intelligent than humans, which I doubt.
It's fair to say then, that English has greater complexity of meaning, yet is not as complex to learn as Humpbacchaen?
I think the aliens would prefer a TEFL teacher to a THFL.
Most probably complexity of sound, as any meaning in any communication (whales, humans, or otherwise) would yet to be deciphered. An alien species, being advanced enough to look across the vastness of space, or perhaps somehow bridge the distance, would probably be looking for the most advanced system of communication, and barring any foreknowledge of meaning, would quite possibly look first for the most advanced from a structural standpoint.
I can't say for sure, because it's been a long time since I read about this stuff, and my brain is like swiss cheese. So there's a good possibility I'm mixing it all up. But if I recall, the structure of the rating system devised from using Information Theory, human speech is at about a 5 on the scale, I think, and whales' "speech" is at a 9.
I believe the rating system is based on the correlation of one "word" to others in a given communication set. For example in an English sentence, you'd look for how strong the correlation is of any given word to the others in that sentence. Of course in the case of whales, it would be the distinguishable sounds in the song and not words. And I have no idea what the criteria are for correlation.
I don't know. Someone with more knowledge of Information Theory and how it's been applied to the study of Communications could shine more light on it. I'm admittedly ignorant of it.